The present invention relates to a toner density sensing apparatus for an electrostatic copying machine.
In an electrostatic copying machine to which the present invention constitutes a substantial improvement, a photoconductive drum is electrostatically charged and radiated with a light image of an original document, thereby forming an electrostatic image through localized photoconduction. A magnetic brush applies a toner substance or mixture to the drum which develops the electrostatic image to form a toner image. The toner image is transferred and fixed to a copy sheet to form a permanent reproduction of the original document.
The toner mixture comprises ferromagnetic carrier particles and black colored non-magnetic toner particles, the toner particles being formed of a resinous powder or the like. The purpose of the carrier particles is to form the magnetic brush which applies the toner particles to the drum. Whereas the toner particles are consumed during copying, the carrier particles are not.
It is therefore necessary to replenish the toner particles in the toner mixture to replace those which are consumed during copying. The supply of replenishment rate must be such that the toner density, herein defined as the proportion of toner particles in the toner mixture, remains constant. If the toner density is too small or too large, the copies will be too light or too dark respectively. Whereas the replenishment rate may be predetermined, such is difficult and generally impossible to determine with acceptable precision in partical application.
For this reason a sensor is necessary which gives an accurate indication of the toner density so that the replenishment rate may be accurately adjusted in an instantaneous manner. Japanese patent publication No. 46-8280 teaches the provision of an electromagnetic coil provided in the toner mixture, the inductance of which corresponds to the toner density since the coil and toner are magnetically coupled. Similarly, Japanese patent publication No. 50-99522 teaches how the current flow through such a coil also corresponds to the toner density.
Although the toner density does correspond to the inductance and current flow through such a coil, measurement thereof for control purposes requires disproportionately complex and expensive circuitry to attain the required precision.
For this reason it has been further proposed in the prior art to provide the coil as one of the frequency determining elements of an oscillator in such a manner that the oscillator fequency corresponds to the inductance of the coil and thereby the toner density. This particular prior art system utilizes a separate local oscillator to produce a reference frequency . The two oscillator frequencies are sampled alternately, and slope detection is utilized to produce a signal corresponding to the toner density. This arrangement is not completely acceptable since the local oscillator adds unnecessarily to the complexity and cost of the circuitry and must be maintained at a precise frequency, thereby requiring expensive crystals control. As another disadvantage, the slope detection circuit is quite sensitive to variations in ambient conditions and is difficult to adjust accurately.